| Oreochromis niloticus belongs to Perciformes and Sparidae,is distributed in Africa,and has been widely introduced into many countries and regions.Oreochromis niloticus has become the most successful fish in the imported fish in mainland China,with the highest yield in the world.In recent years,Oreochromis niloticus has formed a natural group in the upper reaches of the Xijiang River and has become an important fishery resource of the Xijiang River due to artificial release and escape from cultivation.However,the arrival of exotic species such as Oreochromis niloticus will definitely have a great impact on the ecological circle of the upper reaches of the Xijiang River,threatening the living environment of indigenous fish.Therefore,it is particularly important to evaluate the development status of the wild Oreochromis niloticus population in the upper reaches of the Xijiang River in the Pearl River system and to carry out the corresponding fishery resources management.Knowledge of fish population variation and genetic diversity is essential for correct assessment and management of fishery resources.At present,there is still a lack of knowledge about the population variation and genetic diversity of Oreochromis niloticus in the upper reaches of the Xijiang River in the Pearl River system.Therefore,the mtDNA D-loop sequence and rDNA ITS-1 sequence of 122 Oreochromis niloticus samples collected from Nanpan River,Beipan River and Hongshui River in the upper reaches of the Xijiang River were studied by DNA molecular marker technology in this study,and the population variation and genetic diversity status of Oreochromis niloticus in the upper reaches of the Xijiang River were explored,so as to provide theoretical basis for the management and utilization of Oreochromis niloticus population in the upper reaches of the Xijiang River.The main research results are as follows:1.Base composition and variation of Oreochromis niloticus D-loop sequence.In this study,122 Oreochromis niloticus mtDNA D-loop sequences with 879 bp in length were obtained by PCR amplification and DNA sequencing techniques,a total of 18 haplotypes were defined in 121 mtDNA D-loop sequences of Oreochromis niloticus.According to the analysis,the content of base A+T in mtDNA D-loop sequence is higher than that of base C+G,and the difference of base composition among the three river wild Oreochromis niloticus populations is small.A total of 193 mutation sites were found in the mtDNA D-loop sequence,the type of the mutations including transition and transversion sites.The Oreochromis niloticus population of Beipan River has the most mutation sites in all three river's population,which is up to143.The ratio of base transition to transversion was not less than 4.The sequence mutation did not reach the saturation state and further mutation was possible.2.Genetic diversity and population variation of Oreochromis niloticus D-loop sequence.The haplotype diversity index(Hd)and nucleotide diversity index(Pi)of mtDNA D-loop sequence in Oreochromis niloticus population are 0.9700 and 0.6439 respectively.The population has rich genetic diversity,which is generally higher than that of cultivated Oreochromis niloticus.Based on the analysis genetic differentiation index(Fst),genetic distance(D)and other relevant data of the three rivers wild Oreochromis niloticus mtDNA D-loop sequence,the gene exchange of the three rivers wild Oreochromis niloticus population is less,the genetic differentiation level is moderate,a few haplotypes have already differentiated to a certain extent,and there is a tendency to develop to different lineages.Oreochromis niloticus has a relatively stable population size in history and has not seen any obvious expansion.3.Base composition and variation of Oreochromis niloticus ITS-1 sequence.121 Oreochromis niloticus rDNA ITS-1 sequences in the upper reaches of the Xijiang River in the Pearl River system were analyzed and a total of 23 haplotypes were defined.The results showed that there were length polymorphisms in the Oreochromis niloticus rDNA ITS-1 sequences in the three rivers,with 520 bp,536bp and 540 bp sequences,of which 520 bp sequences were the most abundant,accounting for 78.04% of the total.The contents of bases A,T,C and G in rDNA ITS-1 sequence are 14.4%,16.1%,38.8% and 30.6%,respectively.The content of bases C+G is significantly higher than that of A+T.39 mutation points were found in the rDNA ITS-1 sequence of 121 Oreochromis niloticus individuals from three populations,with a base variation rate of 7.5%.4.Genetic diversity and population variation of ITS-1 sequence.The Oreochromis niloticus rDNA ITS-1 sequence in the upper reaches of the Xijiang River in the Pearl River 's haplotype diversity index and nucleotide diversity index are 1.0000 and 0.0320 respectively,which also have high genetic diversity,especially the Hongshui river and Beipan river's population.Based on the analysis of genetic differentiation index(Fst),genetic distance(D)and other relevant data of the three rivers wild Oreochromis niloticus rDNA ITS-1 sequence,the three rivers wild Oreochromis niloticus populations also have less gene exchange,relatively stable size in history and no expansion,which is consistent with the research results of mtDNA D-loop sequence.However,in the genetic differentiation among the three populations,the Fst value of Oreochromis niloticus ITS-1 sequence is less than 0.05,and there is no differentiation among the populations,which is inconsistent with the results of mtDNA D-loop sequence,and may be caused by the different genetic modes of mtDNA and rDNA.To sum up,the population variation of the wild Oreochromis niloticus population in the upper reaches of the Xijiang River in the Pearl River system is relatively poor and its genetic diversity is rich.No obvious differentiation has taken place due to geographical isolation and environmental differences.No obvious expansion has taken place in the population history,but there is a trend of multi-lineage development. |
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